Method of fabricating small spherical pellets



Feb. '10, 1959 H. c. BRASSFIELD EI'AL 2,872,719

METHOD OF FABRICATING SMALL SPHERICAL PELLETS Filed Sept. 25, 1957 INVEN TOR-5. x/i/verz. IAWJJF/flfl BY mez z wax/4w J6 %M a 4M METHOD OFFABRICATING SMALL SPHERICAL PELLETS Henry Clay Brassfield, Fosters,Uhio, and Carl T. Durham, .lr., Roanoke, Va., ass'ignors to the UnitedStates of America as represented by the United States Atomic EnergyCommission Application September 25, 1957, Serial No. 686,268

13 Claims. (Cl. 25-156) This invention deals with a method and apparatusfor the fabrication of small spherical shot type pellets, and inparticular with a method and apparatus for the formation of sphericalpellets from ceramic or refractory materials.

In the prior art, spherical pellet formation is affected by severalmethods, chiefly by dropping from shot towers, by the use of centrifugalforce, or by molding. One great disadvantage inherent in all of theseabove mentioned methods for spherical pellet formation is thefact thatit is extremely difiicult to control the size of the pellets with anygreat degree of exactitude. Another disadvantage is that none of thesemethods is particularly adapted to the production of very smallspherical pellets. The present invention accomplishes spherical pelletformation by a new and radically different method, which method isparticularly applicable to the formation of extremely small sphericalpellets.

An object of this invention is to provide a method and apparatusparticularly adapted to effect the production of large quantities ofsmall spherical pellets conveniently and economically.

A further object of this invention is to provide a meth- 0d andapparatus particularly adapted to efiect the pro- 1 duction of smallspherical pellets, wherein the range of pellet sizes may be easily andautomatically controlled thus insuring a consistently uniform product.

These objects are accomplished by preparing a suspension, or slip, ofpowdered ceramic material and water with a controlled pH, dropping thesuspension through a small capillary into a moving bed ofwater-repellent pow der, drying the spherical drop on the moving bed,separating the spheres from the bed powder, and firing the spheres at1500" C.l800 C. to form a dense body.

For a more full and complete understanding of the subject invention,reference is now made to the drawing of an apparatus designed for thepractice of this invention.

In Figure l; the capillary tube, at the tip of which the drops ofceramic suspension form, is designated as 1, a container for the slip orsuspension which is positioned above and connected to the capillary tubeis designated by 2 and 3 designates a conveyor belt which carries themoving bed of water repellent powder 4. The powder hopper 5 andstationary guides 6, provide for the deposition of this moving bed ofwater repellent powder on the upper surface of the belt. A battery ofinfra red lamps 7, is positioned over the belt and a screening element,8,'is located at the end of the conveyor belt, wherein the sphericalpellets are separated from the water repellent powder. A voltage supplyis designated by 9 and the cable connecting the voltage supply to theslip container as 10.

In operation the water repellent powder is loaded into hopper 5, fromwhich it falls in a thin stream onto the. moving belt 3 which ispositioned below the hopper.. This powder is shaped into a narrow stripby the stationary guides 15. The narrow'bed of water repellent powder isthen carried underneath the capillary tip 1, by the mo- 2,872,719Paftented Feb. 10, 1959 tion of the conveyor belt 3, and as drops ofceramic containing slip form on the tip of the capillary tube 1, theyare deposited on the surface of water repellent powder 4. The waterrepellent properties of the powder cause these small drops to assumespherical shape, and while retaining this spherical shape, the pelletsare carried under the battery of infrared drying lamps 7. Here the wateris evaporated away from the pellets. The slightly damp ceramic spheresare then carried to the separatory screen 8. Here the water repellentpowder rails through the screen while the pellets are retained upon it.The spherical pellets are then collected and fired to produce thefinished product.

The capillary-drop apparatus consists of a plastic container, supportedabove the traveling powder bed, and equipped with a small motor stirrerto maintain uniform dispersion of the suspension of water and ceramicmaterial. The aluminum paddle of the stirrer is electrically insulatedfrom the motor shaft by a plastic coupling, and the shaft is sealed atthe container lid by means of a rubber gasket.

The size of the drop produced is controlled in three ways. The size isdetermined principally by the efiective diameter of the capillary tip;however, this dimension having been fixed, variation may be secured byregulation of gas pressure above the liquid and by application of anelectrical charge to the slip.

The surface of the capillary tip upon which the drop forms should bekept as small as possible. This is accomplished by grinding the tip to anarrow lip. In addition, if the slip is permitted to wet the tip andstem of the capillary, it will creep up the outside wall of the tube anddrops will form on a larger surface. Wetting may be prevented by theapplication of a water-repellent material. No detailed study of therelation of the capillary diameter to drop size has been made, butpractice has shown that internal diameters from 0.012 to 0.018-inchyield drops within the desired size range.

Gas pressure is applied to the slip container primarily to control thedropping rate, but this in turn affects the drop size. The gas is argon,which is first saturated with water to minimize evaporation of water inthe slip and is then led through a pressure regulator to the apparatus.Pressure adjustment is manual, and the normal operating pressure of 0.5to 2.0 p. s. i. is read on a 0-5 p. s. i. gauge.

More precise control of drop size is obtained by imposition of a highvoltage between the slip and the water repellent powder bed. The voltageis applied to the slip,

which is insulated from the ground, by means of a coaxial and areduction of the surface tension of the forming.

drops by the accumulation of positive charges on its sur face. To formthe drops, potentials between 500 and 4000 volts have sufiiced. 7

Many ceramic materials are suitable for treatment by this process,specifically alumina (A1 0 beryllia (BeO), uranium dioxide (U0 andmolybdenum disilicide (Mosi or mixtures of these materials or otherceramic materials. However, the ceramic material actually used must bein a very finely divided state. For example the dried MoSi pelletsproduced by this method will not sinter to a satisfactorily dense bodyon firing unless the MoSihas been ground to an'average particle size of13.0 microns or less.

The creation of a well-formed sphere by the capillary- .dropmethoddepends upon the properties of the ,slip 'or- References Cited inthe file of this patent UNITED STATES PATENTS Cofiin et a1. Nov. 17,1925 Buse Aug. 19, 1930 Hermann Nov. 21, 1939 Hood Feb. 24, 1948Mollring July 24, 1951 Eilbracht et a1. Apr. 30, 1957 Garloni July 16,1957

1. A PROCESS FOR PRODUCING SPHERICAL PELLETS OF MATERIAL, WHICH CONSISTSOF POWDERING THE MATERIAL TO BE PELLETIZED, FORMING A SUSPENSION OF THISPOWDERED MATERIAL IN A LIQUID CARRIER OR VEHICLE, CAUSING SMALL DROPS OFTHIS SUSPENSION TO REST UPON A SURFACE OF LIQUID REPELLENT POWDER,DRYING THE DROPS OF SUSPENSION, AND COLLECTING AND FIRING THE SPHERICALPELLETS THUS FORMED.